The present invention relates to the treatment of ampholytes such as amino acids using an electrodialytic water-splitter. More particularly, the present invention is directed to the purification of ampholytes such as amino acids with the simultaneous production of acids and/or bases.
Reference is made to Condon and Meislich, Introduction To Organic Chemistry, published by Holt, Rinehart and Winston, Inc., pages 472-476 (New York, 1960), for a brief review relating to the properties of amino acids. Amino acids are amphoteric and ionized both as an acid and as a base in aqueous solution. For example, they can be titrated with a strong mineral acid (HCl) or alkali (NaOHl and form salts with either. Ionization with an acid results in formation of cationic species, while ionization with a base results in formation of an anionic form of the ampholyte. Condon discloses that during electrolysis of the solution of amino acid, the cationic form migrates toward the cathode, while the anionic form migrates toward the anode, each at a rate determined by the concentration and mobility. In strongly acidic solutions the cationic form dominates, and the net migration is toward the cathode, while in strongly basic solutions, the net migration is toward the anode.
At some intermediate pH characteristic for each amino acid, most of the acid is in the bipolar form and there is no net migration toward either electrode. The pH at which there is no net migration is known as the "isoelectric point". At the isoelectric point the fraction of amino acid in the bipolar form is a maximum, and its solubility a minimum. Advantage is taken of the differential rates of migration in a method of separating amino acids in proteins, known as electrophoresis.
Electrodialysis at the isoelectric point of amino acid/salt solutions will result in a separation of salt from the amino acid since the amino acid carries no net charge to cause it to migrate in the electric field. However, when the solution is not at the isoelectric point, the amino acid will have a net charge and will migrate through either the anion or cation permeable membranes, depending on the pH. It is not always convenient to adjust the pH to the isoelectric point, and in the case of mixtures of amino acids with widely differing isoelectric points, it may not be possible at all. For example, aspartic acid (ASP) HOOCCH.sub.2 CH(NH.sub.2)COOH, and phenylalanine (PA) C.sub.6 H.sub.5 CH.sub.2 CH(NH.sub.2)COOH, are found in the manufacture of the artificial sweetener aspartame and accumulate in a stream of high salt concentration. The isoelectric point of ASP is 2.98 and PA is 5.91, so a compromise would need to be reached on the proper pH for operation if salt were to be removed by electrodialysis. While PA is more valuable than ASP, ASP is also of substantial value and the loss of either amino acid to the salt would be undesirable.
The separation of amino acids from salts by electrodialysis is a well known process. The advantages of operating with acid on the anode side and base on the cathode side were realized as long ago as 1958, Peers, Electrodialysis Using Ion-exchange Membranes II. Demineralization of Solutions Containing Amino Acids, Journal of Applied Chemistry, 8, pp 59-67, January 1958. Peers discloses, beginning at page 65 the effect of anolyte and catholyte compositions. Peers uses a three-compartment electrodialysis unit wherein there is a center compartment and two side compartments separated from the center compartment by an anion exchange membrane and a cation exchange membrane. A solution of amino acid in salt is fed to the center compartment. Peers discloses that an acidic anolyte compartment results in the least amount of amino acid lost from solution after removal of 70% of sodium chloride present in the solutions. The behavior of the amino acid under circumstances of an acidic anolyte and a basic catholyte compartment is reviewed beginning at page 66 of Peers. Beginning at the first full paragraph at page 67, Peers considers the pH of the feed solution containing salt plus amino acid. Peers indicates that the bulk solution pH is not entirely unimportant if the amino acid concerned is strongly acidic or basic. Peers concludes that the separation of amino acid from sodium chloride has been shown to be improved by operating with relatively low current densities in a combination of acidic anolyte and alkaline catholyte.
Of interest is Smith et al., Electrolytic Desalting With Ion Exchange Membranes, Nature on page 83, Jan. 14, 1956. Also, of interest, is Hara, The Separation of Amino Acid With Ion Exchange Membrane, Bull. Chem. Soc. Japan, Volume 36, No. 11, pages 1373-1376 (1962). This paper discloses a separation of a mixture of amino acids obtained from the hydrolysis of gluten carried out by electrodialysis.
Also, of interest is Hara, Permeability of Acidic Amino Acid Through Anion Exchange Membrane, Bull. Chem. Soc Japan, Volume 36, No. 2, pages 187-194 (1963).
Electrodialytic water-splitting in a two-compartment cell is well known. For example, U.S. Pat. No. 4,391,680 discloses the generation of strongly acidified sodium chloride and aqueous sodium hydroxide by two-compartment water-splitting of aqueous sodium chloride. Three-compartment electrodialytic water-splitters are known in the art. They are disclosed to be comprised of alternating bipolar, anion and cation exchange membranes, thereby forming alternating acid, salt and base compartments. U.S. Ser. No. 235,562 discloses three compartment electrodialytic water-splitters. U.S. Pat. No. 4,740,281 discloses the recovery of acids from materials comprising acid and salt using an electrodialysis apparatus to concentrate the acid followed by the use of an electrodialytic three-compartment water-splitter to separate the acid from the salt.
U.S. Pat. No. 4,608,141 discloses a multi-chamber two-compartment electrodialytic water-splitter and a method for using the same for the basification of aqueous soluble salts. U.S. Pat. No. 4,536,269 discloses a multi-chamber two-compartment electrodialytic water-splitter and a method for using the same for the acidification of aqueous soluble salts. These two patents review the use of two-compartment electrodialytic water-splitters and their use to treat salts.
The staging of two conventional two-compartment electrodialytic water-splitters, whereby the base solution is withdrawn from the base compartment of one two-compartment water-splitter, and is fed through the base compartment of the second two-compartment water splitter, is known. In an attempt to increase the efficiency of bipolar membranes, U.S. Pat. No. 3,111,472 (Oda, et al.) discloses disposing a microporous water permeable cation or neutral membrane in the acid and/or base compartments of the three compartment electrodialytic water-splitter.
Bipolar membranes have been known to be useful for the process of electrodialytic water-splitting to generate an acid and a base for many years (Oda et al., U.S. Pat. No. 2,289,095, Chlanda et al., U.S. Pat. No. 3,787,304, Jenczewski et al., U.S. Pat. No. 4,552,635). Their use in various cell configurations has been reported (Oda et al., Japanese 2023 ('58) reported in Chemical Abstracts 53:11070b., U.S. Pat. No. 4,536,269 and U.S. Pat. No. 4,608,141).
None of the above references disclose separating a amphoteric compound from a salt solution using a bipolar membrane.